This invention relates to the field of ceramic processing. More particularly, this invention relates to a method for making fine particulates of ceramic powders through the reduction of selected ionic species by thermophilic bacteria
In the production of ferrites and many other ceramics, mixed oxide powders are traditionally synthesized by mixing oxides, carbonates, etc., calcining at a high temperature, and milling to reduce particle size. The process is energy and time intensive, frequently hard to control, and sometimes must be done several times before the final product is obtained.
Also, there is a lower practical limit on the particle size that can be achieved by mechanical milling. Grinding ceramic materials to smaller sizes requires progressively more energy due to several reasons, including the greater difficulty in milling smaller particles that have fewer strength limiting defects than coarser particles. Additionally, grinding inefficiencies become significant because of mechanical aspects related to the transfer of useful energy from the milling medium to the particles and to fluid aspects related to the transport of particles through the grinding zone in the mill. Accordingly, for any given milling approach, the required milling time increases as the desired particle size decreases. Generally, contamination of the powder increases with increasing milling time because of abrasion of material from the medium.
Alternatively, chemical routes such as precipitation, sol-gel, and the like can be employed. These processes are generally more costly than calcining, but may yield a product with a higher level of chemical homogeneity and a very fine particle size. The precipitation of iron oxides by aqueous routes generally yields ferric oxide (Fe2O3), which must be reduced (by heating in hydrogen, for example) if a magnetic iron oxide (Fe3O4) is the desired product. This treatment adds more process steps, often including a milling step to break up agglomerates formed during the reduction process.
It has been known for some time that certain bacteria reduce Fe(III) in various geochemical environments. Microbial Fe(III) reduction has been observed primarily in low temperature environments that have been extensively influenced by modern surface biogeochemical processes such as weathering or microbial metabolism(Lovley, 1993; Nealson and Saffarini, 1994). It is also known that certain bacteria such as Desulfovibrio desulfuricans reduce sulfate to sulfide under anaerobic conditions (Rickard, 1969). The formation of some mineral deposits such as magnetite deposits in banded iron formations in both ancient and modern times may be attributed to the action of such bacteria.
Magnetotactic bacteria such as those described by Dunin-Borkowski, et al., (1998) form magnetic nanocrystals within the cell. However, the ratio of product nanocrystals to biomass is relatively low, typically a few nanocrystals per cell.
Several varieties of thermophilic bacteria such as Thermoanerobacter and Thermoanerobium are known to reduce Fe(III) ions as part of their respiration processes. These bacteria have been found in core samples from two geologically and hydrologically separated sedimentary basins, the Taylorsville Basin in Virginia and the Piceance Basin in Colorado. Both the Taylorsville and Piceance Basins have been isolated from surface processes for millions of years. The conditions under which the bacteria were found are summarized below in Table 1.
The differences in thermophiles found in hot springs in comparison with deep subsurface include environmental conditions such as pressure, nutrients and evolution. The actual differences between the microorganisms may be minor; however, thermophiles from hot springs have not yet been demonstrated to produce mixed oxides.
As described by Liu et al., (1997), the bacteria appear to utilize any of several electron donors such as formate, acetate, lactate, pyruvate, or hydrogen, provided that an electron acceptor such as amorphous Fe(III) oxyhydroxide is present. Fe(III) oxyhydroxide is evidently converted to magnetite by the bacteria as a byproduct of their respiration. Magnetite is formed outside the cells in copious amounts of single crystals of well defined size averaging about 60xc2x120 nm, and morphology.
It is also known that thermophilic bacteria reduce other metal ions, notably Cr(VI), Co(III), Ni(III), Mn(IV), and U(VI), (see, for example, Zhang et al., 1996). These studies were directed to the use of thermophilic bacteria to remediate metal-contaminated water and were conducted in cultures in the presence of only a single metal species.
The present invention provides a method to directly produce a wide variety of nanoparticulate mixed oxides, particularly those that are useful in the preparation of ceramics. The method takes advantage of the natural ability of selected thermophilic bacteria to efficiently reduce metal ions in the presence of a suitable electron donor, thereby providing nanoparticulates uniquely suitable for producing mixed oxides as well as selected doped crystalline phases.
Finely divided mixed oxides are frequently used as ceramic colorants and glazes. The oxides of cobalt are particularly well known in these applications. Because the disclosed process can reduce Co(III) to Co(II), it can be adapted to the manufacture of various mixed oxides in which cobalt oxide is the major constituent. Added oxides such as those of Fe, V, Cr, Mn, and Zn may be incorporated to change the color behavior and/or refractoriness analogous to the additions of such dopants to Fe3O4 and Cr2O3. Numerous combinations of mixed oxides can be synthesized by this route, provided only that at least one of the metals in the process can be reduced from a first valence state to a second valence state through the respiration of the bacteria.
Another aspect of the present invention is an apparatus for producing a particulate. The apparatus includes a container and a solution in the container. The solution includes a first reducible metal, a second metal, and a bacterial culture. The bacteria reduce at least a portion of the first reducible metal to form the particulate. The particulate includes at least a portion of the first metal so reduced and at least a portion of the second metal which may or may not be reduced under the culture conditions.
The first metal used in the method or apparatus may include reducible transition metals, such as Fe(III), Cr(VI), Co(III), Ni(III), Mn(IV), U(VI), or other transition elements. The second or dopant metal may include reducible or non-reducible metals, such as Fe(III), Cr(VI), Co(III), Ni(III), Mn(IV), U(VI), Ni(II), Al(III), Zn(II), Mg(II), Mn(II), Cu(II), Co(II), or Pd(II). (The bacteria may comprise thermophilic bacteria.)
Bacteria for use in practicing the disclosed invention are selected from among thermophilic bacteria, preferably those that are typically grown and metabolize under conditions of elevated temperature, about 42xc2x0 C.-65xc2x0 C. Increased pressure may be used; however, the inventors find that under conditions of about 65xc2x0 C. and normal atmospheric pressure conditions, mixed oxide particulates are efficiently produced.